A typical gas turbine engine includes a fan, compressor, combustor, and turbine disposed along a common longitudinal axis. Fuel and compressed air discharged from the compressor are mixed and burned in the combustor. The resulting hot combustion gases (e.g., comprising products of combustion and unburned air) are directed through a conduit section to a turbine section where the gases expand to turn a turbine rotor. In electric power applications, the turbine rotor is coupled to a generator. Power to drive the compressor may be extracted from the turbine rotor.
With the efficiency of a gas turbine engine increasing with operating temperature, it is desirable to increase the temperature of the combustion gases. However, temperature limitations of the materials with which the engine and turbine components are formed limit the operating temperatures. Airfoils of turbine blades and vanes are exemplary. The term blade as used herein refers to a turbine blade or vane having an airfoil. That is, the airfoil may be a part of a rotor (rotatable) blade or a stator (stationary) vane. Due to the high temperature of the combustion gases, airfoils must be cooled during operation in order to preserve the integrity of the components. Commonly, these and other components are cooled by air which is diverted from the compressor and channeled through or along the components. It is also common for components (e.g., nozzles) to be cooled with air bled off of the fan rather than the compressor.
Effective cooling of turbine air-foils requires delivering the relatively cool air to critical regions such as along the trailing edge of a turbine blade or a stationary vane. The associated cooling apertures may, for example, extend between an upstream, relatively high pressure cavity within the airfoil and one of the exterior surfaces of the turbine blade. Blade cavities typically extend in a radial direction with respect to the rotor and stator of the machine.
It is a desire in the art to provide increasingly effective cooling designs and methods which result in more effective cooling with less air. It is also desirable to provide more cooling in order to operate machinery at higher levels of power output. Generally, cooling schemes should provide greater cooling effectiveness to create more uniform heat transfer or greater heat transfer from the airfoil.
Ineffective cooling can result from poor heat transfer characteristics between the cooling fluid and the material to be cooled with the fluid. In the case of airfoils, it is known to establish film cooling along an exterior wall surface. A cooling air film traveling along the surface of an exterior wall can be an effective means for increasing the uniformity of cooling and for insulating the wall from the heat of hot core gases flowing thereby. However, film cooling effectiveness is difficult to maintain in the turbulent environment of a gas turbine.
Consequently, airfoils commonly include internal cooling channels which remove heat from the pressure sidewall and the suction sidewall in order to minimize thermal stresses. Achieving a high cooling efficiency, based on the rate of heat transfer, is an important design consideration in order to minimize the volume of air diverted from the compressor for cooling. By way of comparison, the aforementioned film cooling, providing a film of cooling air along outer surfaces of the air-foil, via holes from internal cooling channels, is somewhat inefficient due to the number of holes needed and the resulting high volume of cooling air diverted from the compressor. Thus, film cooling has been used selectively and in combination with other cooling techniques. It is also known to provide serpentine cooling channels within a component.
However, the relatively narrow trailing edge portion of a gas turbine airfoil may include up to about one third of the total airfoil external surface area. The trailing edge is made relatively thin for aerodynamic efficiency. Consequently, with the trailing edge receiving heat input on two opposing wall surfaces which are relatively close to each other, a relatively high coolant flow rate is desired to provide the requisite rate of heat transfer for maintaining mechanical integrity. In the past, trailing edge cooling channels have been configured in a variety of ways to increase the efficiency of heat transfer. For example U.S. Pat. No. 5,370,499, incorporated herein by reference, discloses use of a mesh structure comprising cooling channels which exit from the trailing edge.
The present invention increases heat transfer efficiency and uniformity of cooling in the trailing edge of a turbine airfoil.
Like reference numbers are used to denote like features throughout the figures.